The present invention relates to a method of manufacturing a heat-shrinkable foam tube for covering a pipe, rod, or the like, to provide heat-insulating, anti-corrosion, waterproofing, and other properties to the metal rod, or the like.
A heat-shrinkable foam tube has been conventionally used for purpose of heat insulating and heat retaining of metal pipes. Heat insulating a metal pipe can be realized, for example, by use of a heat-shrinkable foam tube made of a thermoplastic polymer having a foaming agent as disclosed in Japanese Patent Examined Publication No. Sho. 58-3819. There are basically four conventional methods of manufacturing heat-shrinkable foam tubes, all relating to when and how the foam is generated on the tube. The methods are: foaming after molding; foaming during extrusion; foaming after cross-linking; and foaming during shrinking.
In the foaming after molding method of manufacturing the tube, the foam is provided in advance and applied after the extrusion molding. However, this method requires the additional step of preparing the foam in advance and thereby raises the cost.
In the foaming during extrusion method, foaming is carried out at the time of extrusion, before cross-linking occurs. In the case of the shrinkable tube, however, if foaming is carried out at the time of extrusion, the inner pressure of the tube so varies that it is impossible to carry out stable extrusion foaming, resulting in large variations of the outside diameter of the tube. Generally, in order to make the outer diameter of the tube constant, the tube extrusion is carried out while a constant pressure is applied to the inside of the tube. However, if the extrusion is carried out with foaming agent in the tube, a pressure due to a foaming gas of the foaming agent is exerted inside of the tube, so that it becomes difficult to maintain constant pressure.
In the foaming after cross-linking method of manufacturing a heat-shrinkable foam tube, the foam is activated on the tube after being cross-linked. Therefore, the degree of cross-linking is maintained low to carry-out foaming. If the degree of cross-linking is too high, foaming cannot be carried out; however when cross-linking is low, thermal resistance decreases. Accordingly, the heat-resisting property of a tube manufactured by this method is not sufficient.
Another deficiency of this conventional method is that the outer diameter of the tube varies depending on the expansion ratio of the tube. Therefore, the outer diameter of the tube after shrinkage cannot be estimated while designing the tube.
In the foaming while shrinking method, the foam portion of the tube is formed at the time of the shrinking operation. Generating the foam involves heating the tube to a temperature not lower than about 180.degree. C. for a considerable period of time in order to activate the foaming agent.
As described, the conventional methods of manufacturing heat-shrinkable foam tubes have the following deficiencies:
(a) if the foaming occurs after molding, extra steps are necessary, which increase the cost;
(b) if foaming occurs at the time of tube extrusion, it is impossible to extrude a tube with high dimensional precision.
(c) if foaming occurs after cross-linking, the heat-resisting property is insufficient; and
(d) if foaming occurs at the time of shrinking, forming must be carried out at a high temperature for a long time.
In view of these deficiencies, the present method has been developed to manufacture heat-shrinkable foam tubes.